2. pointer

Computer Science and Software EngineeringUniversity of Wisconsin - Platteville

2. PointerYan Shi

CS/SE2630 Lecture Notes

2

C++ Data Types

structured

array struct union class

address

pointer reference

simple

integral enum

char short int long bool

floating

float double long double

What is reference? simple data type: reference operator & the address of a variable of certain data type int num = 10; int &rNum = num;

Do you remember?— an array or object must be passed as a reference parameter

int nums[10];Student stu;StudentList stuList;…avg = Average(nums);stuList.Add(stu);

Once a reference is created, it cannot be later made to refer another variable/object; it cannot be reseated.

int Average( const int myArray[] );

void Add( const Student& stu );

What is a pointer variable?

A pointer variable is a variable whose value is the address of a location in memory.

Unlike a reference variable, a pointer can redirect to other locations later.

To declare a pointer variable, you must specify the type of value that the pointer will point to.

int *p; // p will hold the address of an int

char *q; // q will hold the address of a charint a, *b; // * is paired with the identifier. // In this case, we have a int variable a and // a pointer of int type b

For a normal variable int num; .

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MemoryAddress identifier

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? num0010

For a normal variable int num; num = 50;

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50 num0010

Pointer int num; num = 50; int *p;

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50 num0010

? p0012A pointer variable contains the memory address of another variable.

Pointer int num; num = 50; int *p; p = &num;

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50 num0010

0010 p0012

& is the reference(address-of )operator.

Pointer int num; num = 50; int *p; p = &num; cout << *p;

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50 num0010

(*) here is the dereference operator.*p is used to access the place p points to. you will see 50 on the screen.

0010 p0012

Pointer int num; num = 50; int *p; p = &num; cout << *p; *p = 100;

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100 num0010

change the value at the address p points to 100

0010 p0012

//direct addressing

//indirect addressing

char ch; ch = ‘A’;

char* q; q = &ch;

*q = ‘Z’; char* p; p = q; // the right side has value 4000 // now p and q both point to ch

Another Example 4000

A Z

ch

5000 6000

4000 4000

q p

NULL pointer Use NULL to initialize pointers that don’t currently

point to anything.— used to initialize pointers— can be converted to pointers of any type— <cstddef>

int *p = NULL;

It is an error to dereference a pointer whose value is NULL. It is the programmer’s job to check for this.

Dynamic Memory Allocation

In the previous example, memory space for num and p are statically allocated — at compile time— from stack memory (activation record and global

variables) Dynamic memory allocation

— at run time— from heap memory (free store: dynamic)

In java, all user-defined types are allocated from heap In C++, use new operator to get data from heap


int *p = new int; ...

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p0010


int *p = new int; ...

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p0010

unnamed dynamicallyallocated integer variable(from heap)

0080


int *p = new int;

With Initialization: int *p = new int(99);

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p0010 0080

0080

The dynamically allocated variablecan only be indirectly addressed through the pointer returned by new.

unnamed dynamicallyallocated integer variable(from heap)

What does new do? It takes a pointer variable, allocates heap memory for it to point, and leaves the address of the assigned

memory in the pointer variable. If there is no more memory, the pointer

variable is set to NULL.

Dynamic Array Using new, now we can dynamically decide the

size of an array.

int size;cin >> size;char *text = new char[size];

Pointers and Arrays C++ arrays are not objects as in Java. They are really just

pointers!char name[30]; // name is actually &name[0]char *np;np = &name[0]; // same as np = name;

C++ allows pointer arithmetic:…cin >> *np;while( *np != ‘/n’ ){ np++; cin >> *np;}

name is a constant pointer. name[i] is the same as *(name + i)

// hope that all names are // shorter than 30 characters// moves np ahead sizeof(char) bytes // and points to the next element.

Pointers and Objects How to declare an object?

Student stu(…); OR

Student *stu = new Student(…);

For the second declaration, we can make a public method call like this:

stu->GetGPA();

// stu is a Student object// located at the stack memory

// stu is a pointer of Student type // located at the heap memory

// This is the same as// (*stu).GetGPA();

Pointers and Objects We can make a dynamic array of objects:

Student * stuList = new Student[n]; In this case, Student must have a default constructor!

An alternative is to make a dynamic array of Student pointers

Student **stuList = new Student*[n]; In this case, no default constructor is needed, but

memory management becomes more complicated.

Memory Leak Memory is allocated but not released causing an application to consume memory reducing the available memory for other applications and eventually causing the system to page virtual memory to the hard drive slowing the application or crashing the application when the computer memory resource limits are reached.

Example:int *p1 = new int;int *p2 = new int(99);*p1 = 10;p2 = p1; // The memory cell p2 originally

// points at now can no longer be // accessed this is called garbage.

Deallocate Memory: delete

delete operator is used to return to the heap a memory location allocated previously by the new operator.

A pointer p is pointing to a dynamically allocated space. When to delete p?— p is about to point to another space;— right before the program exit.

int *p1 = new int;int *p2 = new int(99);*p1 = 10;delete p2; // This prevents memory leak.p2 = p1; …int *a = new int(n);…delete[] a; // deallocate the entire array space.

Enable Memory Leak Detection Visual Studio provides C Run-Time Libraries (CRT) debug heap

functions. To enable:

include in the exact order.

add _CrtDumpMemoryLeaks(); immediately before the program exit.

When you run your program under the debugger, _CrtDumpMemoryLeaks displays memory leak information in the Output window.

#define _CRTDBG_MAP_ALLOC #include <stdlib.h> #include <crtdbg.h>

Dangling Pointer Pointers that do not point to a valid object.

— Dangling pointers arise when an object is deleted or deallocated, without modifying the value of the pointer, so that the pointer still points to the memory location of the deallocated memory.

— If later the program dereferences the (now) dangling pointer, unpredictable behavior may result.

That is why Java introduced automatic garbage collection!

Dangling Pointer Example

8

ptr

-5

ptr2

int* ptr = new int; *ptr = 8; int* ptr2 = new int; *ptr2 = -5; ptr = ptr2;

8 cannot be addressed and thus become a garbage

Dangling Pointer Example

int* ptr = new int; *ptr = 8; int* ptr2 = new int; *ptr2 = -5; ptr = ptr2;

delete ptr2;// ptr is left dangling ptr2 = NULL;

8

ptr

NULL

ptr2

Dangling Pointer Example common mistake: returning address of local data

Both create dangling pointers!— xyz will be deleted after the function call— returned pointer will be pointing to empty slot.

X* foo() { X xyz; ... operate on xyz ... return &xyz;}

char* g() { char str[100]; ... operate on str ... return str; }

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